Although a highly malignant tumor, neuroblastoma can spontaneously regress or differentiate, induction of this differentiation process is a potentially useful therapeutic strategy. The potent glucocorticoid dexamethasone and other agents which generate cyclic AMP will induce morphological differentiation in murine neuroblastoma in culture. Using a clone derived from C-1300 murine neuroblastoma, the effects of glucocorticoid treatment on (a) morphological differentiation and growth inhibition; (b) catecholamine fluorescence and tyrosine hydroxylase activity; and (c) intracellular cyclic AMP levels will be assessed. By injection into syngeneic mice (A/J), cells morphologically differentiated in culture will be observed for their tumorigenic potential. The effect and possible clinical usefulness of dexamethasone on neuroblastoma tumors will be evaluated, using injections of the agent systemically and directly into the tumor. Experiments will also be conducted aimed at more fully comprehending the role of type C viruses in the differentiation process induced by dexamethasone or other agents and in tumorigenesis.